The state-of-the-art microporous pure silica membranes have shown good separation properties in both gas and liquid separations, but suffer from water adsorption at room temperature due to the hydrophilicity of the silica surface. De Vos et al. (J. Membr. Sci. 158, 1999, 277-288; J. Membr. Sci. 143, 1998, 37; EP-A 1089806) developed hydrophobic silica membranes (also referred to as methylated silica membranes) for separation of gasses and liquids and proposed a method for reducing water molecule interaction by incorporation of a precursor containing hydrophobic groups. Methylated silica membranes were further studied for the dehydration by pervaporation of organic solvents by Campaniello et al. (Chem. Commun., 2004, 834-835). They found that the decrease in water flux could be restored by increasing the methyl content (hydrophobicity) of the membranes. Using this approach it was possible to achieve a satisfactory performance up to temperatures of 95° C. However, these membranes are not stable at higher temperatures, which are necessary for separating water from organic solvents. As a result the observed selectivity decreases, leading to failure within a few weeks.
Wang et al. (Chem. Mater. 2004, 16, 1756-1762) describe the synthesis of mesoporous ethylene-silica by acid-catalysed hydrolysis of bis(triethoxysilyl)ethane (BTESE) in the presence of a poly(ethylene oxide) surfactant as a pore former. Similarly, Xia and Mokaya (Micropor. Mesopor. Mater. 2005, 86, 231-242) disclose the synthesis of spherical microporous material containing bis-silylethane bridges by base-catalysed hydrolysis of BTESE in the presence of a cationic surfactant as a pore former.
Lu et al. (J. Am. Chem. Soc. 2000, 122, 5258-5261) describe the preparation of thin mesoporous periodically arrayed films containing bis-silyl-organic bridges, also using surfactants as pore formers. They report calculated pore diameters of 1.8 nm and 2.5 nm for membranes produced using cationic an anionic surfactants, respectively.
Shea and Loy (Chem. Mater. 2001, 13, 3306-3319) present an overview on materials based on bridged polysilsesquioxanes, and provide methods of controlling the properties of the porous materials made. They report that under particular conditions, e.g. long flexible bridges as found in bis(triethoxysilyl)octane (BTESO), and the use of acid catalyst, the porous materials can collapse, leading to dense gels. Further an increase in pore size of gels with increasing length of the alkylene-bridging group was demonstrated for base-catalysed reaction conditions. No report has been made about a material that possesses micropores in the absence of larger mesopores or macropores.
These prior art materials are typically periodic mesoporous organosilicas (PMO), with an average pore size in the mesoporous region with a diameter of >1.5 nm, and normally made in the form of monoliths with typical dimensions in the order of centimetres. Proposed applications are in the field of chromatography. Other applications that have been proposed range from surface modifiers and coatings to catalysts. These materials can be either dense or porous. In general a wide range of pore sizes is observed, and mesopores up to 50 nm coexist with macropores larger than 50 nm. In addition to these pores, micropores smaller than 2 nm may or may not be present. These prior art methods and products do not provide microporous (<2 nm) separation membranes that are sufficiently thermally stable and selective to allow for the continuous and effective separation of gasses or liquids.